1. Field
The present invention relates to fuel cells, in particular to fuel cells in operation of which an alcohol or other low molecular weight fuel is supplied as fuel to the anode region of the cell. Such cells have applications in microfuel cells for electronic and portable electronic components, and also in larger fuel cells for portable and stationary power and the automotive industry.
2. Description of the Related Art
A fuel cell is an electrochemical energy conversion device that converts fuel and oxidant into reaction products, producing electricity and heat in the process. In one example of such a cell, methanol is used as fuel, and air or oxygen as oxidant, and the products of the reaction are carbon dioxide and water. The electrochemical reactions in this cell in operation may be summarised as follows:
AnodeCH3OH + H2O⇄CO2 + 6H+ + 6e−Cathode 3/2O2 + 6H+ + 6e−⇄3H2OOverallCH3OH + 3/2O2⇄CO2 + 2H2O
The methanol fuel and oxidant are fed respectively into catalysing, diffusion-type electrodes separated by an electrolytic membrane which allows the passage of protons from the anode chamber to the cathode chamber to balance the cathode reaction. The electrons generated in the anode chamber flow in an external electrical circuit and are returned to the cathode having provided the power output from the cell. Such fuel cells are known as direct methanol fuel cells (DMFCs).
Direct Methanol Fuel Cells (DMFCs) are extremely useful as they can transform fuel into electricity at a scale that is difficult for other technologies such as the internal combustion engine. The fuel, methanol, is readily transportable and has a high energy density, making DMFCs attractive for use in portable applications and as replacement batteries in portable electronic devices.
Various types of membrane may be used, such as polymer electrolyte membranes (PEMs), comprising for example Nafion™. Fuel cells based on polymer electrolyte membranes (PEM fuel cells) are convenient for portable applications such as portable electronics and automotive technology due to their relatively low temperatures of operation. Further or alternative adaptations to the PEM barrier include the provision of a bimembrane as described in our co-pending application PCT/EP2006/060640.
US-A-2004/0137297 discloses an ion exchange membrane said to be useful for the diaphragm of a direct methanol type fuel cell.
Methanol and other low molecular weight alcohols are convenient fuels for portable fuel cells because their energy density is relatively high, eg, for methanol, six moles of electrons being generated in the electrochemical half cell for every mole of fuel consumed. However, DMFCs typically suffer from crossover effects—methanol is transported across the membrane by diffusion and electro-osmosis. This causes a reduction in the performance of the fuel cell by the effect of methanol being oxidized at the cathode, typically comprising Pt or other noble metal catalyst. Here the methanol is oxidized at the potentials of oxygen reduction. The potential and current are reduced, causing a loss in power density; the open circuit potential is also reduced.
Conventionally, routes to reduce the methanol crossover effect have included:                i) Increasing the membrane thickness—typically 170 μm Nafion is used instead of the more common 50 μm membrane for hydrogen fuel cells. This increases the resistance of the membrane—whilst not completely eliminating the crossover impact.        ii) Introducing layers to restrict methanol crossover within the MEA        iii) Using an alternative membrane to the Nafion-type sulphonated fluoropolymer. Usually these membranes require higher temperatures (>100° C.) to operate effectively, or conduct less well or swell.        iv) Using selective catalysts for the cathode. These are generally poorer catalysts for oxygen reduction than noble metal-containing catalysts such as Pt and Pt-containing catalysts.        
The phenomenon of methanol crossover and potential solutions have been reviewed recently:                International activities in DMFC R&D: status of technologies and potential applications, R Dillon, S Sriinivasan, A S Arico, V Antonucci, J Power Sources, 127, 112 (2004).        M P Hogarth, T R Ralph, Platinum Metal Reviews, 46, 146 (2002).        A Heinzel, V M Barragan, J Power Sources, 84,70 (1999).        V Neburchilov, J Martin, H Wang & J Zhang, J Power Sources, 169, 221 (2007)        
Routes using selective catalysts include the mixed reactant route. For instance, compact mixed reactant (CMR) fuel cells use selective electrode catalysts, and have porous membranes to pass the reactants through the membrane electrode assembly (MEA). In International Patent Application No. WO01/73881 a mixed reactant system is disclosed which includes porous electrodes and electrolyte (if not formed by the reaction mixture). In this arrangement, it is an essential feature that the mixed reactants are exposed to both electrodes. A further example of a mixed reactant system is disclosed in US2004/058203.
In addition to methanol crossover, a further problem suffered by DMFCs is water crossover, where water crosses the polymer electrolyte membrane from the anode to the cathode with methanol.
The methanol and water cross the membrane because of osmotic drag. As protons flow through the membrane they ‘pull’ water and methanol molecules with them.
Conventionally, DMFCs operate using a dilute methanol fuel, typically 1M. This solution is used to protect the MEA, to provide water as fuel for the anode reaction and also to limit the amount of methanol so as to reduce methanol crossover.
However, approximately three molecules cross the membrane per is molecule of methanol. As a result, the methanol concentration in the anolyte will become progressively stronger. To prevent this happening, the water which crosses through the membrane must be returned to the anode to maintain the dilution of the methanol. This requires:                i) the collection of the liquid water and pumping to return it,        ii) or a membrane to restrict the movement of liquid water and provide a pressure to return it to the anode, which reduces performance,        iii) or evaporation of the water, followed by condensation and return to the anolyte. To evaporate the water requires heat and limits the efficiency of the cell.        
Maintaining the anolyte at a 1M concentration requires a control system including apparatus in which pure methanol is fed (using a first pump) to an anolyte mixer where it is mixed with water to form the 1M anolyte. The 1M anolyte is then recirculated (using a second pump) to the anode. In practice, a gas/liquid separation system will also be required.